In the steel industry, surface-treated steel sheets are produced as high value products widely used in automobiles, home appliances, construction, and containers. Surface-treated steel sheets having various features are being required as industry advances.
Various surface treatment techniques are used to improve the corrosion resistance of steel sheets, and most of the surface treatment techniques involve wet processes such as electroplating, hot dipping, chemical conversion treatment, and painting. Zinc has commonly been used as a material for improving the corrosion resistance of steel products, and various methods such as adding other materials or using PVD or CVD processes have been studied and proposed for improving the properties of zinc plating layers and to satisfy increased customer demand for highly corrosion resistant steel products. In addition, as the use of magnesium (Mg) having a high degree of specific strength increases in many industries such as the automobile and electronics industries, there is increasing interest in steel sheets containing magnesium.
Mg steel sheets and Mg-alloy-plated steel sheets such as Zn—Al—Mg—, Zn—Al—Mg—Si—, Zn—Mg—, Mg/Zn—, Al—Mg—, or Al—Mg—Si-plated steel sheets are highly corrosion-resistant; however, while being transported or stored in high-temperature and high-humidity areas, Mg steel sheets or Mg-alloy-plated steel sheets may be blackened by oxygen or moisture contained in the atmosphere (known as a blackening phenomenon).
When a plated steel sheet containing Mg makes contact with moisture, hydroxides and/or oxides of Mg and Zn may be formed on the surface of the plated steel sheet, which are considered to be the cause of a blackening phenomenon. Such a blackening phenomenon may spoil the appearance of a steel sheet to lower the value of the steel sheet, and customers may file claims against vendors providing blackened steel sheets.
Therefore, there have been various efforts to prevent or suppress the blackening of steel sheets having Mg on the surfaces thereof such as an Mg steel sheet and a steel sheet plated with an Mg-containing material.
According to typical methods for preventing or suppressing a blackening phenomenon, Mg steel sheets or Mg-alloy-plated steel sheets may be oiled, anodized, or coated. However, oiling is not suitable for preventing a blackening phenomenon in the case of storing steel sheets for a long period of time.
Anodizing may be used instead of a PVD or CVD process to improve the rate of plating or film forming and thus to improve workability and productivity. In an anodizing process, the surface of a steel sheet is etched to remove oxides, and the etched portion and other exposed portions of the steel sheet are firmly coated with an oxide film (anodic film) to prevent a blackening phenomenon. However, large amounts of strong inorganic acids, toxic to the environment and humans, are used to form such anodic films. In addition, the anodizing process is complicated and time-consuming compared to other processes in a continuous production line, and thus it may be difficult to apply such an anodizing process to a continuous production line.
Steel sheets may be coated with organic films, mostly organic silane films to prevent a blackening phenomenon. However, this method requires a high processing temperature to form inorganic films and a long period of time to dry inorganic films. Therefore, spray coating methods applicable to continuous production lines have been proposed to prevent a blackening phenomenon.
A surface treatment technique using a spray coating method is disclosed in Korean Patent Application Laid-open Publication No. 2007-0082367. In the disclosed technique, an anodic film is formed on an Mg alloy product through ten steps. Except for a product mounting step, a dyeing step, and a drying step, seven steps are actually involved in the surface treatment. However, the disclosed technique is not for steel sheets but for steel parts and is uneconomical because of an excessive amount of steps and a long processing time.
U.S. Pat. No. 7,754,799 discloses a method for improving the chemical resistance of a magnesium alloy steel sheet using titanium chelate, epoxy silane, calcium carbonate treated with paraffin, and zinc oxide treated with organic polysiloxane, non-reactive silicon oil, and silane. Although this method requires a low processing temperature, the drying process time thereof may be relatively long, and organic polysiloxane and silicone oil make it difficult to apply topcoat paint. Therefore, it is difficult to use the disclosed method according to various usages of magnesium steel sheets or alloy steel sheets.
Japanese Patent Application Laid-open Publication No.: 1997-241828 discloses a technique for preventing a blackening phenomenon by dipping a Zn—Mg plated steel sheet in a phosphoric acid pickling solution containing 0.01 wt % to 30 wt % of a phosphoric acid to reduce the concentration of Mg in the surface of a Zn—Mg plating layer to 1% or less and thus to suppress the generation of hydroxides or oxides of magnesium. However, during the treatment using the phosphoric acid pickling solution, the sheen and surface qualities of the Zn—Mg plated steel sheet may be spoiled, and it is necessary to remove sludge remaining on a pickled area of the Zn—Mg plated steel sheet.
Japanese Patent Application Laid-open Publication No.: 1997-143679 discloses a technique of previously blackening the surface of a Zn—Mg plated steel sheet. According to the disclosed technique, Zn and Mg are sequentially deposited on a steel sheet, and the steel sheet is heated to plate the steel sheet with a Zn—Mg alloy film (Zn—Mg plating film) having a predetermined thickness and a single-, double-, or triple-layer structure. Then, the plated steel sheet is left at a temperature as high as 80° C. and in humidity as high as 80% for 10 minutes to 60 minutes so as to modify the surface of the plated steel sheet to Mg(OH)2, Zn, and ZnO. However, according to the disclosed technique, the surface of the Zn—Mg plating film may not be uniformly blackened, and thus the appearance of the steel sheet may be poor. That is, the characteristics of a Zn—Mg plated steel sheet such as a highly sleek and aesthetic appearance may not be obtained. Thus, the value of steel sheet products may be lowered, and the defective rate of steel sheet products may be increased.